that sort of crank failure is usually bad vibration because the crank wheels are not truly balanced, cases out of true or the crank bearings are not getting oil, inspect the bearing retaining plates inside that oil can actually reach the bearings inside, Some very square shoulder bearings can block oil supply. Or if you have a machinist make a sleeve for the bearing - it can be a cause of failure too, unless you are positively sure that oil was getting to the bearing.
When I was rebuilding my workhorse Rx115 - I had to look for the main bearings in the bearing market of Karachi to finally get the slightly rounded shoulder bearings I wanted - the bearing bosses were loose so I had them tig welded then ground and trued, finely made oilways. The crank also needs to be trued one wheel at a time then both as an assembly with a dial indicator on the axle wheels and loctite bearing tite on the big end crank pin, You only have one shot to true the crank within 15 minutes before the glue locks the pin in place.
The crank assy then needs to be pressed in the cases with a puller NOT A HAMMER like the ullu and stupid bike mechanics in Pakistan use.
that workhorse bike served me without complain EVER for about a decade. The engine never vibrated, never sounded noisy and never ever shorted out a spark plug (I was using the original oil system though with Emarat 2 stroke oil). I used to replace its spark plug every year.
It takes precision to make an engine reliable.
